In the semiconductor industry, chemical mechanical polishing (CMP) is used to selectively remove portions of a film from a semiconductor wafer by rotating the wafer against a polishing pad on a platen (or rotating the pad against the wafer, or both) with a controlled amount of pressure in the presence of a chemically reactive slurry. Overpolishing ( removing too much ) or underpolishing (removing too little) of a film results in scrapping or rework of the wafer, which can be very expensive. Various methods have been employed to detect when the desired endpoint for removal has been reached, and the polishing should be stopped. One such method described in copending patent application Ser. No. 08/419,206, Pat. No. 5,559,428, uses a sensor which can be located near the back of the wafer during the polishing process. As the polishing process proceeds, the sensor generates a signal corresponding to the film thickness, and indicates when polishing should be stopped.
IPEC/Westech of Phoenix, AZ manufactures a Westech chemical mechanical polishing apparatus (polisher) that is commonly used in the semiconductor industry. The part of the Westech polisher that holds the wafer and rotates it (upside down) against a polishing pad as shown in FIG. 1 includes a bottom plate 100, a carrier drive ring 102, a T-bar 104, and a carrier cover 106. A wafer to be polished is held upside down against the bottom side 101 of bottom plate 100 by a vacuum created using an insert film (hereafter called insert pad) with holes (not shown). The entire assembly rotates and is held by a robot arm of the polisher (not shown) connected through opening 108 to T-bar 104.
When using the sensor of copending application Ser. No. 08/419,206 filed Apr. 10, 1995 entitled "In-Situ Monitoring of the Change in Thickness of Films" assigned to the present assignee with the Westech polisher, positioning it near the back of the wafer necessitates locating it somewhere in the wafer carrier. This creates several problems. First, a rotary means of signal coupling is needed to get the signal from the sensor out to monitoring apparatus. Second, the signal coupling must be mechanically robust to minimize wear. Third, the signal must be shielded from the effects of noise created by the rotating carrier. Fourth, exposure (both for contacts and the sensor) to the chemically corrosive slurry must be minimized. Finally, any modifications made to the carrier should be easily retrofitted to an existing Westech polisher to minimize changes to other parts of the system such as the robot arm.
Thus there remains a need for rotary signal coupling for use in chemical mechanical polishing endpoint detection that solves the above-listed problems and is suitable for use in a Westech polisher.